In some communications applications, it is desirable to receive a main signal and its diversity signal and to communicate both signals to a second location where one or the other signal is selected as the preferred signal for reception. In some applications it is desirable that the communication to the second location use an optical fiber as the communications medium.
For example, a microbase station in a cellular communication system might use a diversity antenna system located anywhere from several meters to several hundred meters away from the microbase station. The diversity antenna system would include an antenna element for receiving a main RF signal and another element for receiving a diversity RF signal corresponding to the main signal. The two RF signals would have the same center frequency. The two signals could be provided by, for example, a polarization diversity antenna system in which a horizontally polarized and a vertically polarized signal are both received and provided to the microbase station. The two signals could also be provided by a spatial diversity antenna system, located away from a microbase station, in which two antennas are physically separated, each receiving a signal traveling from the source by a different path, and both signals are to be provided to the microbase station.
With the diversity antenna system removed from the microbase station where the receiver components are located that select either the main signal or the diversity signal, both RF signals must be transmitted from the remote antenna system to the microbase station. It is often desirable to do this using optical fiber as the transmission medium; this of course requires that the RF signals modulate an optical carrier signal.
In propagating within the optical fiber, the main signal and diversity signal of either a spatial diversity antenna system or a polarization diversity antenna system are essentially indistinguishable, because the two signals have the same center frequency and the features that provide diveristy for propagation in air, polarization or path taken, are not available within the optical fiber. Therefore, measures must be taken to keep the two signals distinct. One way to keep the signals distinct is to shift one or both signals in RF frequency before modulating the optical carrier. Then after demodulating the optical carrier, the shifted RF signal or signals are converted back to their original RF frequency. In doing this back conversion, however, the back-converted signal is often distorted.
What is needed is a way to transmit a main signal and its diversity signal, at essentially the same center frequency as the main signal, along a single optical fiber in a way that does not cause distortion, and can be implemented without unduly complex hardware.